After reading your response to my questions and viewing other responses, I too agree that it's a lack of fertilizer. Usually when seeding, I apply a good quality Starter Fertilizer on the same day that I seed. Three weeks later, I apply a second dose of it. Three weeks after that, I look at the lawn and if it's not coming in too well at that time, I'll apply another dose of the Starter. I'll continue to do this until the lawn is full and thick. If it does become full and thick by the time I'm ready for the 3rd application, I'll switch over to a regular fertilizer or combo product (whichever may be needed at that time.)